1. Field of Invention
The present invention relates to a polysilicon manufacturing process. More particularly, the present invention relates to a low temperature polysilicon manufacturing process.
2. Description of Related Art
The use of polysilicon film in the fabrication of transistor products is very popular in recent years. Polysilicon film transistors are often used as switching devices in display unit. Compared with amorphous film transistors, polysilicon film transistors have higher carrier migration rate. Hence, a polysilicon film transistor can be smaller and can serve as a peripheral driver circuit, especially in high-resolution display. However, the growth of polysilicon film demands a relatively higher temperature so that not only is the thermal budget increased, but a more heat-resistant substrate is also required so that a higher production cost will result.
Conventionally, a polysilicon film layer is formed by one of the following methods:
1. Solid-Phase Crystallization (SPC): Conventional low-pressure chemical vapor deposition is used to form an amorphous film over a substrate. A high temperature annealing operation is carried out to grow crystals in the amorphous silicon film so that ultimately the amorphous silicon is transformed into a polysilicon film. A temperature of at least 600.degree. C. is used in the annealing step. PA1 2. Metal-Induced Crystallization (MIC): A metallic film is formed over an amorphous layer at a relatively low temperature so that silicon in the amorphous silicon film and metal in the metallic film react to form a silicide film. Thereafter, a portion of the silicon atoms in the silicide layer is leached out to form crystals. Hence, crystallization temperature changes according to temperature of the silicide material. In general, the crystallization temperature is about 500.degree. C. PA1 3. Excimer Laser Annealing (ELA): Excimer laser is beamed onto an amorphous silicon film at room temperature so that temperature of film rises rapidly to the boiling point of silicon almost instantaneously and then seeds the amorphous silicon film with crystal nucleus. Ultimately, the seed nuclei formed inside the amorphous silicon film grow in size to form a polysilicon film.
In the aforementioned methods, method one needs to use a high processing temperature of about 600.degree. C. Hence, a higher thermal budget must be required and a higher heat resistant substrate must be used. In the second method, although a lower temperature can be used, residual metal in the polysilicon film often produces deep trap that may interference with conductive carriers. In the third method, due to short burst of energy by the laser beam, the silicon film may re-solidify so quickly that there is insufficient time to crystal out. In addition, the laser method can form a polysilicon film having a thickness from several tens of nanometers to a few hundred nanometers only. Moreover, uniformity of the polysilicon film is hard to control.